A cognitive radio is able to sense the spectral environment over a wide frequency band and exploit this information to opportunistically provide wireless links that best meet a user's communication requirements. What is unique to cognitive radio operation is the requirement that the radio is able to sense the environment over huge swath of spectrum and adapt to it since the radio does not have primary rights to any pre-assigned frequencies. That is, it is necessary to detect the existence of signals of primary users dynamically.
Energy (EG) detection is a major and basic signal detection method (sensing algorithm). Unlike coherent detection, energy detection does not need any information of the signal to be detected and is robust to unknown multipath fading. However, energy detection is vulnerable to the noise uncertainty, because the method relies on the knowledge of accurate noise power. There are several sources of uncertainty: (1) non-linearity of components; (2) thermal noise in components (non-uniform, time-varying); (3) noise due to transmissions by other users (unintentional (close-by) or intentional (far-away)). Hence, in practice, it is very difficult (virtually impossible) to obtain the accurate noise power.